Allulose epimerase variant, method of producing the same, and method of producing allulose using the same

ABSTRACT

The present invention provides a novel allulose epimerase variant and various uses thereof, in which glycine (Gly), an amino acid residue at position 216 of the amino acid sequence of wild-type D-allulose 3-epimerase derived from Flavonifractor plautii, is substituted with serine (Ser). The novel allulose epimerase variant according to the present invention has a higher conversion activity of fructose to allulose compared to the wild-type D-allulose 3-epimerase derived from Flavonifractor plautii, and in particular, it has excellent thermal stability under high temperature conditions of 60° C. or higher, so that the enzyme conversion reaction is performed at the industrial level for mass production of allulose to prevent contamination, to shorten production time, and to reduce production cost.

TECHNICAL FIELD

The present invention relates to an allulose epimerase variant and the like, and more particularly, an allulose epimerase variant and various inventions derived therefrom with an improved conversion rate of fructose to allulose and thermal stability compared to D-allulose 3-epimerase derived from Flavomfractor plautii.

BACKGROUND ART

D-allulose is an epimer of the third carbon of fructose, which is also called D-psicose. D-allulose has 70% sweetness compared to sugar (Oshima 2006), but only 0.3% energy so that it is a functional monosaccharide that can be applied as a low-calorie sweetener in dietetic foods (Matsuo et al. 2002). Further, D-allulose has the function of absorbing glucose and inhibiting blood sugar so that it can be applied to food products for diabetics, health food products, etc. Since it can suppress the accumulation of abdominal fat through inhibition of enzyme activity involved in lipid synthesis in the liver, it can be used in various functional foods such as health foods (Matsuo et al. 2001; Iida et al. 2008; Hayashi et al. 2010; Hossain et al. 2011).

Due to the above characteristics, allulose is a good source to replace sugar. However, it belongs to rare sugars, which are monosaccharides that are extremely rare in nature. Thus, a method of efficiently producing allulose is required for application to the food industry. The conventional method of producing allulose was mainly performed through a chemical process. Billik et al. proposed a method of converting fructose into allulose using the catalytic action of molybdate ions. McDonald produced allulose from 1,2:4,5-di-δ-isopropylidene-beta-D-fructopyranose by a three-step chemical treatment process. Further, Doner produced allulose by heating fructose together with ethanol and trimethylamine. However, although these chemical production methods are expensive, there is a disadvantage in that their efficiency is low, and by-products are significantly generated.

As a biological method of producing allulose, a method of producing allulose has been proposed from galactitol, D-tagatose, D-thalitol, or the like using the cellular reaction of microorganisms (Ken Izumori). However, this method is difficult to apply to industrial production since the substrate belongs to rare sugars. The most efficient way for industrialization is to find an enzyme that converts fructose into allulose among the group of D-ketose 3-epimerase. Previously published contents include the production of allulose from fructose using D-tagatose 3-epimerase expressed in the transformed E. coli after inserting D-tagatose 3-epimerase derived from Clostridium cellulolyticum H (10) (Mu et al. 2011), Agrobacterium tumefaciens (Kim et al. 2006), Pseudomonas cichorii (Itoh at al. 1994), and Rhizobium sphaeroides (Zhang et al. 2009) into E. coli and transforming the same.

Regarding the technology for producing allulose from fructose using an enzyme, Korean Patent Publication No. 10-0744479 discloses a method for producing allulose using an allulose epimerase derived from Agrobacterium tumefaciens. Further, Korean Patent Publication No. 10-0832339 discloses a method of converting fructose into allulose using Sinorhizobium YB-58 KCTC 10983BP, which has the activity of converting fructose into allulose. Korea Patent Publication No. 10-1106253 discloses E. coli including a polynucleotide encoding the allulose 3-epimerase of Agrobacterium tumefaciens C58, which has an activity to catalyze the conversion of fructose into allulose, and a method of producing allulose from fructose using the same. Korean Patent Publication No. 10-1339443 discloses a ketose 3-epimerase derived from microorganisms belonging to the genus Rhizobium and a method of converting fructose to allulose using the same. Korean Patent Publication No. 10-1318422 discloses a D-allulose 3-epimerase derived from Clostridium scindens and a method of producing allulose from fructose using the same. Korean Patent Publication No. 10-1473918 discloses a D-allulose 3-epimerase derived from Flavonifractor plautii and a method of producing allulose from fructose using the same.

However, the wild-type D-allulose 3-epimerase derived from microorganisms does not have a high conversion rate of fructose into allulose and is particularly not suitable for industrialization due to poor thermal stability under optimal conditions of activation temperature. Therefore, there is a need to develop a novel D-allulose 3-epimerase variant with improved conversion rate or thermal stability of fructose to allulose compared to the wild-type D-allulose 3-epimerase derived from microorganisms. Regarding the D-allulose 3-epimerase variant, Korean Patent Laid-Open Publication No. 10-2014-0021974 discloses D-allulose 3-epimerase derived from Treponema primitia ZAS-1, which induces mutations at the gene level and shows a fast conversion rate to allulose and stability at high temperatures. Korean Patent Publication No. 10-1203856 discloses an allulose epimerase variant with improved thermal stability obtained by variation of a wild-type allulose epimerase derived from Agrobacterium tumefaciens.

DISCLOSURE Technical Problem

The present invention is derived from the background of the prior art, and the first object of the present invention is to provide a novel D-allulose 3-epimerase variant with the improved conversion rate of fructose to allulose and thermal stability compared to the wild-type D-allulose 3-epimerase derived from Flavonifractor plautii.

A second object of the present invention is to provide a method of producing a novel D-allulose 3-epimerase variant or to provide various elements necessary for producing a novel D-allulose 3-epimerase variant.

A third object of the present invention is to provide a method of producing allulose from fructose or to provide various elements necessary for producing allulose from fructose.

Technical Solution

The applicants of the present invention have applied for a patent, and the invention of producing allulose from fructose using wild-type D-allulose 3-epimerase derived from Flavonifractor plautii has been registered (See Korea Patent Publication No. 10-1473918 (2014.12.11)). The wild-type D-allulose 3-epimerase derived from Flavonifractor plautii shows the maximum activity for the conversion of fructose to allulose in a pH range of about 6.5 to 7.0 and a temperature of about 62 to 66° C. As the reaction time elapses under the optimum condition of temperature, the enzyme activity rapidly decreases. Thus, its utilization is limited in the industrialization stage for mass production of allulose. Further, since the enzyme conversion reaction for mass production of allulose is generally carried out at high temperatures to prevent contamination, it is necessary to significantly improve thermal stability in order to apply the wild-type D-allulose 3-epimerase derived from Flavonifractor plautii to industrialization. The applicants of the present invention recognize the inherent issues of wild-type D-allulose 3-epimerase derived from Flavonifractor plautii, and utilize protein structure prediction technology to confirm that when the amino acid sequence at a specific position of the amino acid sequence of wild-type D-allulose 3-epimerase is substituted, the conversion rate of fructose into allulose and thermal stability was improved, thereby completing the present invention.

In order to achieve the first object, the present invention provides an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5.

In order to achieve the second object, the present invention provides a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5. Further, the present invention provides a recombinant vector including a polynucleotide encoding an allulose epimerization enzyme variant consisting of the amino acid sequence represented by SEQ ID NO: 5. Further, the present invention provides a recombinant strain transformed by a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5. Further, the present invention provides a method for producing an allulose epimerase variant, the method including the steps of: culturing a recombinant strain transformed by a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 to express the allulose epimerase variant, and separating the allulose epimerase variant from the lysate of the recombinant strain in which the allulose epimerase variant is expressed.

In order to achieve the third object, the present invention provides a composition for producing allulose, the composition including the allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5. Further, the present invention provides a composition for producing allulose, the composition including a recombinant strain transformed by a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5, the culture of the recombinant strain or the lysate of the recombinant strain. Further, the present invention provides a method for producing allulose, the method including the step of: reacting fructose with an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a composition including the allulose epimerase variant. Further, the present invention provides a method for producing allulose, the method including the step of: reacting fructose with the recombinant strain transformed by a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5, the culture of the recombinant strain, the lysate of the recombinant strain, or a composition including any one or more thereof.

Advantageous Effects

The novel allulose epimerase variant according to the present invention has a higher conversion activity of fructose to allulose compared to the wild-type D-allulose 3-epimerase derived from Flavonifractor plautii, and in particular, it has excellent thermal stability under high temperature conditions of 60° C. or higher, so that the enzyme conversion reaction is performed at the industrial level for mass production of allulose to prevent contamination, to shorten production time, and to reduce production cost.

DESCRIPTION OF DRAWINGS

FIG. 1 shows the position of a total of five amino acid residues selected as candidates for substitution to improve the conversion rate of fructose to allulose and thermal stability of D-allulose 3-epimerase derived from Flavonifractor plautii by the inventors of the present invention.

DETAILED DESCRIPTION OF EMBODIMENT

Hereinafter, the present invention is described in detail.

One aspect of the present invention relates to a novel D-allulose 3-epimerase variant capable of converting fructose to allulose (hereinafter referred to as “allulose epimerase variant”). The allulose epimerase variant is a wild-type D-allulose 3-epimerase derived from Flavonifractor plautii of which glycine (Gly), an amino acid residue at position 216 of the amino acid sequence is substituted with serine (Ser) and has high conversion activity of fructose to allulose and particularly has excellent thermal stability in high temperature conditions of 60° C. or higher compared to wild-type D-allulose 3-epimerase. The allulose epimerase variant may be obtained by a method in which a polynucleotide encoding a wild-type D-allulose 3-epimerase derived from Flavonifractor plautii (consisting of the nucleotide sequence represented by SEQ ID NO: 7) is used as a template, PCR is performed using an oligonucleotide having a predetermined nucleotide sequence as a primer pair, then, an overlap extentention PCR is performed using the pair of amplified variant fragments as a template and using the oligonucleotide into which the sequence of the restriction enzyme recognition site is introduced as a primer, then a polynucleotide fragment encoding the amino acid sequence of the allulose epimerase variant is inserted into the expression vector to prepare a recombinant expression vector, then the host strain is transformed with the recombinant expression vector to prepare a recombinant strain, and then the recombinant strain is cultured and expressed. The allulose epimerase variant according to the present invention consists of the amino acid sequence represented by SEQ ID NO: 5, but the equivalent range of the allulose epimerase variant according to the present invention is not limited thereto. For example, as long as the activity of converting fructose to allulose and thermal stability at a high temperature of 60° C. or higher are maintained, some of the amino acids represented by SEQ ID NO: 5 may be substituted, inserted and/or deleted. The amino acid substitution is preferably made by conservative amino acid replacement, which does not change the properties of the protein. Further, modification of the amino acid may be performed by glycosylation, acetylation, phosphorylation, or the like. Further, the equivalent range of the allulose epimerase variant according to the present invention includes proteins with increased structural stability against heat, pH, etc. due to mutation or modification in amino acid sequence or increased activity for the conversion of fructose to allulose. Further, the equivalent range of the allulose epimerase variant according to the present invention may include an amino acid sequence having 70% or more, 80% or more, 90% or more, 95% or more, or 99% or more homology with the amino acid sequence represented by SEQ ID NO: 5.

Another aspect of the present invention relates to a method for producing a novel allulose epimerase variant or to various elements required to produce a novel allulose epimerase variant. Various elements necessary to produce the novel allulose epimerase variant include polynucleotides, primer pairs, recombinant vectors, recombinant strains, and the like.

The polynucleotide is a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5, and preferably consisting of the nucleotide sequence represented by SEQ ID NO: 11. The term “polynucleotide” as used herein refers to all polyribonucleotides (RNA) or polydeoxyribonucleotides (DNA) that are non-modified or modified. The polynucleotide includes single-stranded or double-stranded DNA, DNA as a mixture of single-stranded and double-stranded regions, single-stranded or double-stranded RNA, RNA as a mixture of single-stranded and double-stranded regions, or hybrid molecules thereof, but are not limited thereto. Further, the equivalent range of the polynucleotide encoding the allulose epimerase variant includes a sequence having substantial homology to the nucleotide sequence represented by SEQ ID NO: 11. The substantial homology is determined by aligning the nucleotide sequence represented by SEQ ID NO: 11 and any other sequence so as to correspond as much as possible, and analyzing the sequences, and means that any of the other sequences above have 70% or more, 90% or more, or 98% or more sequence homology with the nucleotide sequence represented by SEQ ID NO: 11. Those of ordinary skill in the art will readily understand that polynucleotide encoding allulose epimerase variants having the same activity within the range having the substantial homology may be prepared by substituting, adding, or deleting one or more bases of the nucleotide sequence of the polynucleotide using gene recombination techniques, etc. known in the art. Such homology comparison may be performed by calculating the homology between two or more sequences as a percentage (%) using a commercially available computer program.

Further, the primer pair is for synthesizing a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5, and is preferably composed of a forward primer and a reverse primer having the following nucleotide sequence.

Forward primer sequence (5′→3′) GCTGGGGCATTTCCACGTGAGCGAGAACAACCGCCGCCCCG Reverse primer sequence (5′→3′) CGGGGCGGCGGTTGTTCTCGCTCACGTGGAAATGCCCCAGC

Further, the recombinant vector includes a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5. The recombinant vector may be provided in the form of inserting a polynucleotide encoding an allulose epimerase variant into a cloning vector or an expression vector using a known standard method. The term “cloning vector” used in the present invention is defined as a material capable of carrying and reproducing a DNA fragment into a host cell. In the present invention, the cloning vector may further include a polyadenylation signal, a transcription termination sequence, and a multiple cloning site. In this case, the multiple cloning site includes at least one endonuclease restriction enzyme restriction site. In addition, the cloning vector may further include a promoter. For example, in the present invention, the polynucleotide encoding the allulose epimerase variant may be located upstream of a polyadenylation signal and a transcription termination sequence. In addition, the term “expression vector” used in the present invention is defined as a DNA sequence necessary for transcription and translation of the cloned DNA in a suitable host. Further, the term “expression vector” used in the present invention refers to a genetic construct including essential regulatory elements operably linked to the insert so that the insert is expressed if it is present in the cells of an individual. The expression vector may be produced and purified using standard recombinant DNA techniques. The type of expression vector is not particularly limited as long as it has a function of expressing a desired gene in various host cells of prokaryotic and eukaryotic cells and preparing a desired protein. However, it is preferably a promoter showing powerful activity and a vector capable of producing a large amount of a foreign protein in a form similar to that of the natural state while having strong expression power. The expression vector preferably contains at least a promoter, a start codon, a gene encoding a desired protein, and a stop codon terminator. In addition, it may appropriately include DNA encoding the signal peptide, additional expression control sequences, untranslated regions on the 5′ and 3′ sides of the desired gene, a selection marker region, a replicable unit, or the like. The term “promoter” means a minimal sequence sufficient to direct transcription. Further, the promoter may include a promoter configuration sufficient to express a cell type-specific or regulatable promoter-dependent gene induced by an external signal or agent, and these configurations may be located at the 5′ or 3′ portion of the gene. These promoters include both conservative and inducible promoters. The promoter sequence may be derived from prokaryote, eukaryote, or virus. The term “operably linked” used herein means that one function is regulated by another by polynucleotide sequence association on a single polynucleotide. For example, if the promoter is capable of regulating the expression of the coding sequence (i.e., the coding sequence is under the transcriptional regulation of the promoter), the promoter is linked and operated to the coding sequence, or if the ribosome binding site is positioned to facilitate translation, the ribosome binding site is linked and operated to the coding sequence. The coding sequence may be operated by linking to a regulatory sequence in the sense or antisense direction. The recombinant vector according to the present invention is preferably an expression vector.

Further, the recombinant strain is transformed with a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5. The term “recombinant strain” used herein refers to a cell transformed by introducing a polynucleotide encoding one or more target proteins or an expression vector having the same into a host cell. Methods for preparing transformants by introducing the expression vector into host cells include transient transfection, microinjection, transduction, cell fusion, calcium phosphate precipitation, liposome mediated transfection, DEAE dextran-mediated transfection, polybrene-mediated transfection, electroporation, electroinjection, a chemical treatment method such as PEG, a method using a gene gun, a heat shock method, and the like, but are not limited thereto. The host cells that may be transformed with the expression vector in the present invention are not greatly limited, as long as they are known in the art, such as prokaryotic cells, plant cells, insect cells, and animal cells, and preferably a host having high DNA introduction efficiency and high expression efficiency of the introduced DNA. For example, the host cell may be E. coli. The E. coli may include BL21, JM109, K-12, LE392, RR1, DH5a, W3110, or the like, but is not limited thereto. In addition, the host cell may be a strain selected from the group consisting of Bacillus strains such as Bacillus subtilis and Bacillus thuringiensis, Coryne bacterial strains such as Corynebacterium glutamicum, Salmonella strains such as Salmonella typhimurium, other Serratia marcescens, and Enterobacteriaceae strains such as various Pseudomonas species.

Further, the method for producing an allulose epimerase variant includes the steps of: culturing the recombinant strain transformed with a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including a polynucleotide encoding an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 to express the allulose epimerase variant; and separating the psicose epimerase from the lysate of the recombinant strain in which the allulose epimerase variant is expressed. Expression of the protein by the host cell may be induced by using lactose, isopropyl-1-thio-β-D-galactopyranoside (IPTG), as an inducing factor, etc., and the induction time may be adjusted to maximize the amount of protein. In the present invention, the allulose epimerase variant may be recovered from the lysate of the recombinant strain. The cells used in protein expression may be lysed by various physical or chemical means such as freeze-thawing repetition, sonication, mechanical breakage, or a cell lytic agent and may be isolated or purified by a general biochemical isolation technique (Sambrook et al., Molecular Cloning: A laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, 1989; Deuscher, M., Guide to Protein Purification Methods Enzymology, Vol. 182. Academic Press. Inc., San Diego, Calif., 1990). For example, the method of isolating or purifying the protein expressed by the host cell includes electrophoresis, centrifugation, gel filtration, precipitation, dialysis, chromatography (ion exchange chromatography, affinity chromatography, immunosorbent affinity chromatography, reverse-phase HPLC, and gel permeation HPLC), isoelectricity focus, and various modified or complex methods thereof, but is not limited thereto. Meanwhile, in the present invention, the isolating of the allulose epimerase from the lysate of the recombinant strain may be preferably performed by affinity chromatography using a peptide tag. As the peptide tag, various known tags such as a HA tag, a FLAG tag, a His tag, a biotin carboxyl carrier protein (BCCP), a c-myc tag, a V5 tag, a glutathione-S-transferase (GST), or a maltose binding protein (MBP) may be used and among them, the His tag is preferably used. The His-tagging protein is specifically trapped on a column of a nickel-nitrilotriacetic acid (Ni-NTA) resin and may be released by EDTA or imidazole.

Yet another aspect of the present invention relates to a method of producing allulose from fructose or various elements required for producing the allulose from the fructose. As various elements required for producing the allulose from the fructose, there is a composition for producing the allulose.

An example of the composition for producing the allulose includes an allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5. Further, another example of the composition for producing the allulose includes a recombinant strain which is transformed by the polynucleotide encoding the allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including the polynucleotide encoding the allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5, a culture of the recombinant strain, or a lysate of the recombinant strain. In this case, preferably, the composition for producing the allulose may further include one or more kinds selected from the group consisting of manganese ions, nickel ions, and cobalt ions, and more preferably, may further include nickel ions or cobalt ions. The novel allulose epimerase variant according to the present invention has a metalloenzyme characteristic in which activation is adjusted by a metal ion and performs the reaction by the enzyme in a presence of a specific metal ion such as nickel ions or cobalt ions to increase a production yield of the allulose.

Further, an example of the method of producing the allulose from the fructose includes reacting the fructose with a composition including the recombinant strain which is transformed by the polynucleotide encoding the allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5 or a recombinant vector including the polynucleotide encoding the allulose epimerase variant consisting of the amino acid sequence represented by SEQ ID NO: 5, a culture of the recombinant strain, a lysate of the recombinant strain or a composition including one or more thereof. Further, the method of producing the allulose from the fructose may additionally include adding metal ions, and a kind of metal ion is as described above. As an example, the metal ion may be added to the fructose that is a substrate or added to a mixture of the enzyme variant and the fructose. Further, as another example, the metal ion may be added to a carrier immobilized with the enzyme variant (before adding the fructose), added to a mixture of the carrier immobilized with the enzyme variant and the fructose (after adding the fructose), or added in a form of the mixture with the fructose when the fructose is added. In the case of using the recombinant strain, the metal ion may be added in the culture or the culturing may be performed in a culture medium added with the metal ion. Further, in the method of producing the allulose from the fructose, the allulose epimerase variant or the recombinant strain is preferably immobilized in the carrier. The carrier may create an environment in which the activity of the immobilized enzyme may be maintained for a long amount of time and may be selected from all known carriers which may be used for enzyme immobilization. For example, sodium alginate may be used as the carrier. The sodium alginate is a natural colloidal polysaccharide abundant in the cell walls of algae and consists of β-D-mannuronic acid and α-L-gluronic acid. In terms of the content thereof, the sodium alginate is formed by randomly forming a β-1,4 bond and the strain or the enzyme is stably immobilized, and thus it is advantageous to have excellent allulose yield. As an example, in order to further promote the yield of the allulose, a sodium alginate solution at a concentration of 1.5 to 4.0% (w/v) (for example, an aqueous sodium alginate solution), preferably a sodium alginate solution at a concentration of about 2.5% (w/v) may be used for immobilizing the recombinant strain. Further, in the method of producing the allulose from the fructose, the reaction temperature is in the range of 60° C. to 70° C., preferably 60° C. to 67° C., and more preferably 62° C. to 65° C. when considering the stability and maximum activity of the enzyme variant, and the reaction pH is in the range of 6.5 to 8, preferably 6.5 to 7.5, and more preferably 6.5 to 7. Further, in the method of producing the allulose from the fructose, the concentration of the fructose is not particularly limited thereto, but preferably 1% to 75% (w/w) and more preferably 4 to 35% (w/w) based on the entire reactant when considering productivity and economics. Further, in the method of producing the allulose from the fructose, the concentration of the used enzyme variant may be 0.001 mg/ml to 0.1 mg/ml, preferably 0.01 mg/ml to 0.1 mg/ml, and more preferably 0.02 mg/ml to 0.05 mg/ml based on the entire reactant. Further, in the case of producing the allulose from the fructose by using the recombinant strain, the host strain of the recombinant strain is preferably a cytologically safe strain. The cytologically safe strain means a generally accepted as safe (GRAS) class strain, which is generally accepted as safe and for example, may be selected from a Saccharomyces sp., Bacillus sp. Corynebacterium sp. strain, and the like. These strains are industrial microorganisms which produce chemical materials having various uses in fields of feed, medicines, and the like. These strains have a strain characteristic which is easily used for gene manipulation and mass culture and has high stability under various process conditions. Further, these strains have a relatively hard cell membrane structure as compared with other bacteria to have a biological characteristic in which these strains are present in a stable state even under high osmotic pressure caused by high sugar concentration, etc. The particular examples of the generally accepted as safe (GRAS) class strain include Saccharomyces cerevisiae, Bacillus subtilis, Corynebacterium glutamicum, and the like.

Hereinafter, the present invention will be described in more detail by Examples. However, the following Examples are only intended to clearly illustrate the technical features of the present invention and do not limit the protection scope of the present invention.

Example 1: Search for Amino Acid Substitution Positions for Improving Conversion Rate and Thermal Stability of D-Allulose 3-Epimerase

In order to improve the conversion rate of fructose to allulose and thermal stability of D-allulose 3-epimerase derived from Flavontfractor plautii, amino acid substitution positions were selected based on protein structure prediction. The D-allulose 3-epimerase (or D-psicose 3-epimerase) derived from Flavontfractor plautii is disclosed in a Korean registered patent No. 10-1473918 (2014.12.11) previously filed and registered by the applicant of the present invention. The D-allulose 3-epimerase derived from Flavonifractor plautii consists of the amino acid sequence represented by SEQ ID NO: 1, has maximum activity for the conversion of fructose to allulose under neutral pH conditions, and is capable of mass-producing allulose from fructose with a high yield in a short time, but there is a problem in that the activity decreases rapidly when the enzyme reaction is carried out under high temperature conditions to prevent contamination, etc. In order to improve the conversion rate of fructose to allulose and thermal stability of D-allulose 3-epimerase derived from Flavontfractor plautii, the inventors of the present invention predicted the protein structure using a homology modeling technique based on the amino acid sequence of the D-allulose 3-epimerase derived from Flavonifractor plautii, to seach for amino acid substitution positions. The homology modeling was performed using a Robetta server, and protein structure prediction and analysis were performed using software programs such as Coot, PyMol, and UCSF Chimera. The three-dimensional structural model analysis of the D-allulose 3-epimerase was performed to select a total of five amino acid residue positions that are expected to be related to the improvement of the conversion rate and thermal stability when the chemical bond is changed and the amino acid residues to be substituted at the positions. FIG. 1 shows the position of a total of five amino acid residues selected as candidates for substitution to improve the conversion rate of fructose to allulose and thermal stability of D-allulose 3-epimerase derived from Flavontfractor plautii by the inventors of the present invention. In FIG. 1, “I21” represents isoleucine (Ile) present at position 21 of the amino acid sequence represented by SEQ ID NO: 1, “A82” represents alanine (Ala) present at position 82 of the amino acid sequence represented by SEQ ID NO: 1, “L133” represents leucine (Leu) present at position 133 of the amino acid sequence represented by SEQ ID NO: 1, “G216” represents glycine (Gly) present at position 216 of the amino acid sequence represented by SEQ ID NO: 1, and “A276” represents an alanine (Ala) present at position 276 of the amino acid sequence represented by SEQ ID NO: 1. In addition, as described below, an enzyme variant I21C consisting of the amino acid sequence represented by SEQ ID NO: 2 was prepared by substituting cysteine Cys) for isoleucine (Ile) present at the position 21 of the amino acid sequence represented by SEQ ID NO: 1. An enzyme variant A82R consisting of the amino acid sequence represented by SEQ ID NO: 3 was prepared by substituting arginine (Arg) for alanine (Ala) present at the position 82 of the amino acid sequence represented by SEQ ID NO: 1. An enzyme variant L133D consisting of the amino acid sequence represented by SEQ ID NO: 4 was prepared by substituting aspartic acid (Asp) for leucine (Leu) present at the position 133 of the amino acid sequence represented by SEQ ID NO: 1. An enzyme variant G216S consisting of the amino acid sequence represented by SEQ ID NO: 5 was prepared by substituting serine (Ser) for glycine (Gly) present at the position 216 of the amino acid sequence represented by SEQ ID NO: 1. An enzyme variant A276R consisting of the amino acid sequence represented by SEQ ID NO: 6 was prepared by substituting arginine (Arg) for alanine (Ala) present at the position 276 of the amino acid sequence represented by SEQ ID NO: 1.

Example 2: Preparation of Recombinant Vector and Recombinant Strain for Overexpression of D-Allulose 3-Epimerase Variant Derived from Flavonfractor Plautii

The fragments of polynucleotide encoding the amino acid sequence of five enzyme variants (I21C, A82R, L133D, G216S, and A276R) were produced using the overlap extension polymerase chain reaction method based on the wild-type polynucleotide of the allulose epimerase derived from Flavonifractor plautii.

First, a PCR reaction was performed using the primers shown in Table 1 below in order to prepare a gene encoding an allulose epimerase variant derived from Flavonifractor plautii. Specifically, 1 pM of the oligonucleotide as a primer in Table 1 and 100 ng of wild-type polynucleotide (SEQ ID NO: 7) of allulose epimerase variant derived from Flavonifractor plautii used as a template were mixed to the reaction solution to which 100 μM of deoxynucleotide triphosphate (dATP, dCTP, dGTP, dTTP) was added. PCR reaction was then performed in 25 cycles to 30 cycles in the presence of 1 unit of pfu-X DNA polymerase mixture (Binoneer) using a Thermocycler (TP600, TAKARA BIO Inc., JAPAN).

TABLE 1 Description of primer Primer nucleotide sequence (5′→3′) I21C Forward primer ACTGGGACGAGATTGCATACTGCCCCCTGATGGAGAAGCTGGC I21C Reverse primer GCCAGCTTCTCCATCAGGGGGCAGTATGCAATCTCGTCCCAGT A82R Forward primer ACCTGGCCAGCGACGATCCGCGCGTGCGGGAGAACGGCATCCG A82R Reverse primer CGGATGCCGTTCTCCCGCACGCGCGGATCGTCGCTGGCCAGGT L133D Forward primer AGAAGCGCCGCAAGGAGGAGGATGCCCTGGAGTCCATGTCCCG L133D Reverse primer CGGGACATGGACTCCAGGGCATCCTCCTCCTTGCGGCGCTTCT G216S Forward primer GCTGGGGCATTTCCACGTGAGCGAGAACAACCGCCGCCCCG G216S Reverse primer CGGGGCGGCGGTTGTTCTCGCTCACGTGGAAATGCCCCAGC A276R Forward primer CAGCGGCGGGGCCGGGGAGCGCGGGCTGGACGAGATGGCGGG A276R Reverse primer CCCGCCATCTCGTCCAGCCCGCGCTCCCCGGCCCCGCCGCTG

After amplifying the variant fragments through primer combination, the overlap extention PCR was performed using each pair as a template and using an oligonucleotide into which the sequence of the NdeI and XhoI restriction enzyme recognition sites was introduced as a primer to finally produce polynucleotide fragments encoding the amino acid sequence of five enzyme variants (I21C, A82R, L133D, G216S, and A276R). Table 2 below shows the nucleotide sequence of the primers used to introduce the sequence of the restriction enzyme recognition site.

TABLE 2 Description Primer of primer nucleotide sequence (5′→3′) NdeI Forward GCATGCCATATGAACCCGATTGGAATGCA primer XhoI Reverse GCATGCCTCGAGCGCGGTCAGCTCCTTGA primer GGA

The nucleotide sequence represented by SEQ ID NO: 8 represents a polynucleotide fragment encoding the amino acid sequence of enzyme variant I21C, the nucleotide sequence represented by SEQ ID NO: 9 represents a polynucleotide fragment encoding the amino acid sequence of enzyme variant A82R, the nucleotide sequence represented by SEQ ID NO: 10 represents a polynucleotide fragment encoding the amino acid sequence of enzyme variant L133D, the nucleotide sequence represented by SEQ ID NO: 11 represents a polynucleotide fragment encoding the amino acid sequence of enzyme variant G216S, and the nucleotide sequence represented by SEQ ID NO: 12 represents a polynucleotide fragment encoding the amino acid sequence of enzyme variant A276R. The nucleotide sequences of SEQ ID NOs: 8 to 12 consist of nucleotide sequences that directly correspond to the amino acid sequence of the enzyme variant, and the sequence of the restriction enzyme recognition site was excluded for convenience.

Thereafter, the prepared polynucleotide fragments were inserted into the same restriction enzyme site of the expression vector pET28a (Novagen) using restriction enzymes NdeI and XhoI to prepare five types of recombinant expression vectors. In addition, a heat shock method (See Sambrook and Russell: Molecular Cloning.) was used to introduce a recombinant expression vector into E. coli BL21 (DE3) (Invitrogen) to transform, thereby producing five recombinant E. coli. A 60% glycerin solution was added to the produced recombinant E. coli and stored frozen at −70° C. before culturing for enzyme expression.

Example 3: Expression and Purification of D-Allulose 3-Epimerase Variant Derived from Flavonifractor Plautii

1 ml of recombinant E. coli prepared in Example 2 was inoculated into a 1 L flask containing 150 ml of protein expression medium having the composition of the following Table 3 (based on 1 L of medium), and they were cultured with a shaking incubator at 32° C. for 24 hours while maintaining the shaking conditions of 32° C. and 140 rpm. Overexpression of the allulose epimerase variant was induced with 1% lactose contained in the medium.

TABLE 3 Medium component Amount Glycerol 9.0 weight % Lactose 1.0 weight % (NH₄)₂SO₄ 0.6 weight % KH₂PO₄ 0.5 weight % YPA 1.5 weight % MgSO₄ 0.1 weight % FeSO₄ 10.0 ppm MnSO₄ 10.0 ppm ZnSO₄ 10.0 ppm antifoaming agent (Neorin) 1.0 drop

Thereafter, the overexpressed allulose epimerase variant was isolated by the following method. First, the culture solution of the recombinant E. coli was centrifuged at 4100×g and 4° C. for about 15 minutes to remove the supernatant, and the strain cells of the recombinant E. coli were recovered. Then, the recovered strain cells of recombinant E. coli cells were suspended in a lysis buffer (containing 50 mM potassium phosphate monobasic, 300 mM potassium chloride, and 5 mM imidazole) and treated with an ultrasonic disruptor (sonicator) to disrupt the cells. Thereafter, the cell lysate was centrifuged at 15,814×g and 4° C. for about 10 minutes to remove the cell pellet, and only the supernatant was recovered. Then, a purified enzyme solution containing an allulose epimerase variant was separated from the supernatant recovered using a histidine tag (His-tag) affinity chromatography column and a desalting column.

Example 4: Confirmation of Conversion Rate of Fructose to Allulose and Thermal Stability of d-Allulose 3-Epimerase Variant

In order to confirm the conversion rate of fructose to allulose and thermal stability of the wild-type and variant of the allulose epimerase derived from Flavonifractor plautii, the enzyme was exposed to high temperature for a certain period of time, and then the degree to which the conversion activity of fructose to allulose decreases was analyzed. Specifically, the purified enzyme solution (enzyme concentration 1 mg/ml) obtained in Example 3 was stored and heated in a constant temperature water bath at 62° C. for 0 hr, 1 hr, and 2 hr. Then, the heat-treated purified enzyme solution was added to a 50 mM PIPES buffer solution (pH 7.0) containing 4% (w/w) fructose and metal ions of 1 mM nickel sulfate (NiSO₄) so that the enzyme concentration became 25 μg/ml. They were reacted for 25 minutes in a temperature condition of 70° C. and a shaking condition of 120 rpm using a shaking constant temperature water bath (VS-1205SW1, Vision Science). Thereafter, the temperature of the reaction product was lowered to 4° C. to stop the reaction, and the supernatant was recovered by centrifugation under conditions of 16,600×g and 4° C. Then, the allulose concentration and fructose concentration in the supernatant were measured using a high-performance liquid chromatography (HPLC) system (SP930D pump, Youngrin equipment; MIDAS automatic sample injector, Spark Holland) and a refractive index detector (2414 refractive index detector, Waters) equipped with a sugar analysis column (Benson, USA). The conversion rate of fructose to allulose was calculated from the measured results, and the conversion rate was used as an index of enzyme activity.

Table 4 below shows the conversion rate of fructose to allulose according to heat treatment conditions when the wild-type and variant of the allulose epimerase derived from Flavonifractor plautii were heat-treated.

TABLE 4 Conversion rate of fructose to allulose by heat treatment time of wild Heat type allulose epimerase and variants thereof (%) treatment Wild time at 62° C. type I21C A82R L133D G216S A276R 0 hr 17.6 13.5 16.5 17.2 17.9 17.8 1 hr 5.2 1.3 3.4 4.8 13.3 4.1 2 hr 2.3 0.3 2.1 2.2 8.5 2.2

As shown in Table 4 above, the conversion activity of fructose to allulose of I21C, A82R, and L133D among the allulose epimerase variants derived from Flavonifractor plautii was lower regardless of heat treatment compared to the wild type of allulose epimerase derived from Flavonifractor plautii. In addition, compared to the wild type of allulose epimerase, the allulose epimerase variant A276R had a slightly higher conversion activity of fructose to allulose when heat treatment was not performed, but the conversion activity of fructose to allulose was significantly decreased by heat treatment. Meanwhile, the allulose epimerase variant G216S showed higher conversion activity of fructose to allulose regardless of heat treatment compared to the wild type of allulose epimerase and particularly showed significantly improved thermal stability.

As described above, the present invention has been described through the above embodiments, but the present invention is not necessarily limited thereto, and various modifications can be made without departing from the scope and spirit of the present invention. Accordingly, the scope of protection of the present invention should be construed as including all embodiments falling within the scope of the claims appended to the present invention. 

1. An allulose epimerase variant consisting of the amino acid sequence of SEQ ID NO:
 5. 2. A polynucleotide encoding the allulose epimerase variant of claim
 1. 3. The polynucleotide according to claim 2, wherein the polynucleotide includes a nucleotide sequence of SEQ ID NO:
 11. 4. A recombinant vector including the polynucleotide of claim
 2. 5. A recombinant strain which is transformed by the polynucleotide of claim
 2. 6. A method for producing an allulose epimerase variant, the method comprising: culturing the recombinant strain of claim 5 to express the allulose epimerase variant; and separating the allulose epimerase variant from a lysate of the recombinant strain in which the allulose epimerase variant is expressed.
 7. A composition for producing allulose, the composition comprising the allulose epimerase variant of claim
 1. 8. A composition for producing allulose, the composition comprising the recombinant strain of claim 5, a culture of the recombinant strain, or a lysate of the recombinant strain.
 9. A method for producing allulose, the method comprising: reacting fructose with the allulose epimerase variant of claim 1 or a composition including the allulose epimerase variant.
 10. A method for producing allulose, the method comprising: reacting fructose with the recombinant strain of claim 5, a culture of the recombinant strain, a lysate of the recombinant strain, or a composition including any one or more thereof.
 11. The recombinant strain of claim 5, wherein the polynucleotide includes a nucleotide sequence of SEQ ID NO:
 11. 12. The recombinant strain of claim 5, wherein the polynucleotide is included in a recombinant vector. 